UK Edition
Liz Truss has vowed that "desperate" Vladimir Putin will be defeated in Ukraine after the Russian president threatened nuclear war.
The Prime Minister said that the world was witnessing a "decisive moment in the history of freedom" and that 2022 was "the story of freedom fighting back”, as she addressed the United Nations General Assembly for the first time.
Her speech followed a warning from Joe Biden to the Russian president that "nuclear war cannot be won and must never be fought".
Ms Truss said that Moscow’s invasion of Ukraine had been a "catastrophic failure" and condemned the Russian president for using “barbarous" tactics.
Putin, in a televised address to the Russian people, earlier made a clear threat to use nuclear weapons.
He said: "In case of a threat against our country’s territorial integrity, we will certainly use all means at our disposal to protect Russia and our people. This is not a bluff."
The Russian leader ordered a partial mobilisation of 300,000 reservists in an attempt to turn the tide of the war.
It was the first such mobilisation by Russia since the Second World War, triggering a rush for the border and protests across the country.
Referendums on joining Russia are set to be held in four occupied areas of Ukraine later this week. The Kremlin is expected to declare that those territories have been annexed.
It came as five Britons held by Russian-backed forces in eastern Ukraine were released in a prisoner swap deal negotiated by Saudi Arabia.
They included Aiden Aslin and Shaun Pinner who were sentenced to death after being captured.
Ms Truss said it was "hugely welcome news" and thanked Volodymyr Zelensky, the Ukrainian president, "for his efforts" to return them to their families.
At the United Nations, Ms Truss held her first meeting as Prime Minister with Mr Biden, who told her: "You're our closest ally in the world."
The US president joined the Prime Minister in forcefully condemning Putin's "overt, reckless and irresponsible" nuclear threats - and issued a stark warning to the Kremlin.
He said the Russian leader had "shamelessly violated the core tenets" of the United Nations Charter.
Mr Biden said Russia is “making irresponsible threats to use nuclear weapons”, adding: “Now, Russia is calling up more soldiers to join the fight, and the Kremlin is organising sham referendums.
"The world should see these outrageous acts for what they are. No one threatened Russia and no one other than Russia sought conflict."
He said the war had been "chosen by one man" and was intended to "extinguish Ukraine's right to exist".
Mr Biden said: "Whoever you are, wherever you live, whatever you believe, that should make your blood run cold."
A White House official said Mr Biden's remarks were intended to be a "firm rebuke" of Putin.
Other leaders refused to be cowed by the Russian leader's threats, effectively calling his bluff.
Ms Truss accused him of making "sabre-rattling threats" while "desperately trying to justify his catastrophic failures".
She said: "He is doubling down by sending even more reservists to a terrible fate. This will not work. The international alliance is strong. Ukraine is strong.
"The Ukrainians are not just defending their own country, they are defending our values and the security of the whole world.
"We won’t rest until Ukraine prevails. This is a decisive moment in our history, in the history of this organisation [the United Nations], and in the history of freedom."
A UK diplomatic source said Putin's threats were being taken "very seriously".
The source said: “The threat either of using tactical nuclear weapons, which would be tragic, or using bigger nuclear weapons is very real."
But the source also noted that Putin was "shaking" when he delivered his address.
Other Western leaders gathered in New York vowed not to recognise the sham referendums and characterised Putin's escalation as a sign of weakness.
Jens Stoltenberg, the Nato secretary-general, said in response to Putin's threats: "The answer is not to step down and to stop supporting Ukraine. The answer, if anything, is to step up and further support Ukraine.
"We will make sure that there is no misunderstanding in Moscow about the seriousness of using nuclear weapons.
"We have been so clear in our communications with Russia about the unprecedented consequences, about the fact that the nuclear war cannot be won by Russia."
He said the only way to end the war was to "prove that President Putin will not win on the battlefield".
Putin's escalation followed mounting casualties and setbacks for Russian forces as they were driven back by a Ukrainian counter-offensive this month.
Following his mobilisation announcement, young Russian men began trying to leave the country.
Plane tickets to countries that do not require a visa, like Turkey and Belarus, quickly sold out.
There were protests in dozens of cities across Russia, some turning violent.
In Moscow, protesters chanted: "Send Putin to the trenches." In the city of Novosibirsk, a young Russian man was filmed yelling: "I don’t want to die for Putin."
As of Wednesday night, 700 people had been arrested by Russian authorities in more than 35 cities.
At the United Nations, Mr Biden said the Russian threats were not the only "disturbing trend" in relation to nuclear weapons.
He said: "China is conducting a concerning nuclear build-up without any transparency."
Mr Biden added: "The United States is clear - we will not allow Iran to acquire a nuclear weapon.
"I continue to believe diplomacy is the best way to achieve this outcome. We cannot let the world now slide backwards."
In their first bilateral meeting, Ms Truss and Mr Biden spoke for 75 minutes - 15 minutes longer than scheduled.
A Downing Street spokesman said the pair agreed to redouble efforts to end energy dependence on Russia.
"The leaders condemned Putin’s recent belligerent statements on Ukraine," he said.
"They agreed his actions highlight the need for allies to continue their economic and military support to Ukraine.
"Both leaders discussed the increased threat posed by autocracies to our values and the international system. The Prime Minister updated President Biden on the UK’s plans to update the Integrated Review to ensure the UK is fully equipped to tackle the evolving challenge from countries like China and Russia.
"The leaders stressed the need to end over-reliance on authoritarian states in terms of our energy, technology and manufacturing supply chains.
“They agreed to redouble bilateral efforts to reduce energy dependence on Russia and to increase the amount of renewable and other forms of energy flowing from democratic states."